1. Field of the Invention
The present invention relates generally to improvements in audio signal processing, and more specifically to a method of expanding a frequency range of a digital audio signal without increasing a sampling rate which has been utilized for converting an original analog audio signal to the digital audio signal.
2. Description of the Related Art
With the advent of recent developments in digital signal processing, it is often the case that an analog audio signal such as a speech signal is converted into a digital signal and then is stored, for later use, in an appropriate storage medium such a RAM (random access memory), a magnetic tape, etc.
Before turning to the present invention it is deemed advantageous to briefly discuss a known method wherein a digital audio signal stored in storage medium is read out thereof for driving a speaker after being converted into the corresponding analog signal.
FIG. 1 is a diagram schematically showing such a known technique. The arrangement of FIG. 1 includes a plurality of functional blocks together with three sketches each of which schematically illustrates spectrum distribution of a digital signal appearing at the functional blocks.
As shown in FIG. 1, an analog-to-digital (A/D) converter 10 is supplied with an analog signal such as a speech signal. The A/D converter 10 samples the incoming analog signal and then quantizes the sampled values as is well known in the art. A digital audio signal thus obtained in stored in a suitable storage medium 12 such as a RAM. Representing a sampling rate as f.sub.5, a sampling theorem tells us that the digital signal stored in the storage medium 12 has a frequency spectrum (frequency range) extending from 0 Hz to f.sub.5 /2.
Therefore, if the A/D converter 10 samples the incoming audio signal at a sampling rate of 8 kHz, then the digital signal stored has a frequency spectrum ranging from 0 Hz to 4 kHz (=f.sub.5 /2). Further, the digital signal stored in the storage medium includes aliasing noises inherently incurred during the sampling, as is known in the art.
A digital audio signal retrieved from the storage medium 12 is applied to a digital signal processor 14. The spectrum of the digital signal is schematically shown in a sketch (A) of FIG. 1. As shown, the digital audio signal (hatched portion) has a frequency range from 0 Hz to f.sub.5 /2 and accompanying aliasing noises. The spectra of the digital signal and the noises are symmetrical with respect to the frequency f.sub.5 /2.
The digital signal processor 14 operates such as to shift the aliasing noises toward a higher frequency range. In other words, the processor 14 moves only the aliasing noises away from f.sub.5 /2. Thereafter, the output of the digital signal processor 14 is applied to a digital-to-analog converter 16 wherein the digital signal is converted into the corresponding analog. Subsequently, an analog low-pass filter 18 allows the audio signal (hatched portion) to pass therethrough. Thus, the aliasing noises can be blocked at the low-pass filter 18. The reproduced analog signal is applied to a speaker 22 via a speaker driver 20 which includes an audio frequency amplifier as is known in the art.
It has been assumed that the digital audio signal has the frequency range from 0 Hz to 4 kHz. If It is desired to reproduce a sound at the speaker 22 in a more natural manner, the frequency range of the stored digital signal should be expanded up to a higher frequency (for example, about 8 kHz). This implies that the sampling rate should be increased to 16 kHz (8 kHz.times.2).
It is sometimes desirable to expand an audio frequency range without increasing a sampling rate. However, no technique has been proposed for effectively expanding a digital audio frequency range without increasing a sampling rate.